1. The Field of the Invention
The present invention relates to artificial joints, and in particular to an artificial inter-vertebral disc for replacement of damaged spinal discs. The present invention relates to an improved artificial inter-vertebral disc for both total disc replacement and for nuclear replacement.
2. State of the Art
Artificial joints are increasingly becoming more common for the medical treatment of degenerated boney joints. Joints may become damaged due to accidents, diseases, aging, etc., and are often replaced when the pain is sufficient, or when natural motion of the joint is sufficiently impaired. Artificial joints commonly replace the tissue between adjoining bones, and may often replace the ends of the two adjoining bones which form the joint.
In replacing a joint, there are generally several desirable outcomes to be achieved. These outcomes include: stability, load bearing capability, natural motion preservation, pain relief, and reduced failure rates and reduction in catastrophic failure. Due to the complexity of the human spine, stability has been a very difficult parameter to address. Often this instability manifests itself as additional wear and premature failure of the artificial joint or supporting physiological structures, adjacent segment/joint degeneration, and exacerbating the pain and disability of the patient.
A number of artificial discs which are presently available tend to lack the stability of the natural spine. Many total disc replacement devices (TDR) are of the “ball in cup” or “ball in trough” design. One of the problems of these particular designs is that the TDR requires the surrounding tissues and structures (ligaments and joints) to provide support and stability. Due to the physical geometry of these designs, the further the spine is moved from the “neutral position” the more the artificial joint has a tendency to continue moving in that direction, thus applying unnatural stress on the surrounding tissues and structures and requiring greater forces to return the joint to the “neutral position.” Over time, the constantly applied and increased loads required to operate the artificial joint may lead to damage to the muscles, connected tissues and adjacent structures of the spine, exacerbating the pain and hampering proper movement of the spine. It has also been discovered that, due to the instability of the replaced disc, the spine can develop scoliosis, or curvature, which tends to lead to additional deterioration of the tissues associated with the spine, such as failure of adjacent joints.
The neutral position for a joint is the normal resting position for the joint, and is typically in the middle of the range of motion for a spinal joint. For a typical spine, two adjacent vertebral bodies have endplates which are approximately parallel in the neutral position.
Another parameter that must also be controlled is the ability to mimic the natural kinematic motion of the spine. Many joints in the human body can be adequately approximated by simple joints such as a hinge or a ball in socket. Because of the complex construct of the spinal joint, it cannot be approximated by simple joints. Many prior artificial discs allow the vertebrae to move in a pivotal motion having symmetrical movements. The differences in movement between a natural joint and an artificial joint can cause undesirable effects on the surrounding muscle and tissue. This can cause a degeneration and inability to properly move and control the artificial joint accentuating the instability of the artificial joint, and may accelerate further joint problems.
There is a need for an artificial joint that is more energetically stable with the inherent tendency to return the joint to a “neutral position” in order to reduce the stress and fatigue on the surrounding tissues and structures. Additionally, there is additional need for the artificial joint to more accurately match the natural kinematic motion of the spine to reduce stress and fatigue again on the surrounding tissues and structures. These are but two parameters important to designing a successful spinal disc replacement.